Generally, laser interference lithography is a technique for conducting exposure to light by using an interference pattern occurring in an overlap region of light with high coherence, which has several traveling wave vectors. Namely, the laser interference lithography is a technique for exposing an interference pattern formed in a light overlap region to a photoresist layer and then developing it. The laser interference lithography is recently spotlighted since it allows to realize a large sub-micrometer pattern in an easy and inexpensive way. The laser interference lithography has a weak point in that it just allows formation of a regular pattern. However, since most patterns demanded in the nano technology have standardized regular patterns, the above weak point does not cause much trouble.
Basic principle of the laser interference lithography can be understood by means of the interference of electromagnetic waves, made in a region where two parallel lights E1 and E2 with different traveling directions are overlapped, as shown in FIG. 1. The parallel lights E1 and E2 are expressed on the assumption that the lights has no phase difference in a direction orthogonal to their traveling directions. The parallel lines of E1 and E2 exhibit the same light phase.
As well known in the art, a wave causes constructive interference at a position with the same phase and destructive interference at a position with different phases. Since light is a kind of wave, the same principle is identically applied thereto. E1 and E2 can be expressed as in the following equation 1.E1(r)=Aej{right arrow over (k)}r·r E2(r)=aej{right arrow over (k)}s·r  Equation 1
where A and a are intensity of electromagnetic wave, and Kr and Ks are wave vectors of E1 and E2.
Meanwhile, the intensity of light is proportional to square of the magnitude of electromagnetic wave, so the intensity profile of light in an overlap region can be expressed as in the following equation 2.
                                                                        I                ⁡                                  (                  r                  )                                            =                            ⁢                                                                                                            E                      ⁢                                                                                          ⁢                      1                                        +                                          E                      ⁢                                                                                          ⁢                      2                                                                                        2                                                                                        =                            ⁢                                                                                                                A                                                              2                                    +                                                                                  a                                                              2                                    +                                      2                    ⁢                                                                A                                                              ⁢                    a                                                  ❘                                  cos                  [                                                                                    (                                                                                                            k                              r                                                        →                                                    -                                                                                    k                              s                                                        →                                                                          )                                            ·                      r                                        +                    ϕ                                                                                                          Equation        ⁢                                  ⁢        2            
Seeing the equation 2, it would be found that the intensity profile of light in a region where E1 and E2 are overlapped is expressed as a periodic function such as a trigonometrical function. Here, the period can be calculated from the equation 2 as in the following equation 3.
                    Λ        =                  λ                      2            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                              Equation        ⁢                                  ⁢        3            
Seeing the equation 3, it would be understood that the period in the overlap region has a close relation with interference angle and wavelength.
The laser interference lithography uses the phenomenon that the intensity profile of light is locally changed in a region where two laser lights cause constructive interference, so a repeated fine pattern can be obtained if a photoresist layer is exposed to an overlapped interference pattern region and then developed. According to the equation 3, it could be understood that a minimum pitch of the fine pattern formable by the laser interference lithography is ½ of the laser wavelength.
FIG. 2 is a graph showing a change of resolution (pitch) of a fine pattern according to an interference angle of laser in case a laser source used in the laser interference lithography is 266 nm in wavelength.
As shown in FIG. 2, it would be understood that the pitch of fine pattern is decreased as an interference angle of laser is increased. It would be understood that the pitch of fine pattern converges to 133 nm. This number is a pitch of fine line width obtainable when the interference angle of laser is 90 degrees, which is corresponding to ½ of the laser wavelength. However, in case the interference angle is 90 degrees, the laser is guided in parallel with the upper surface of the photoresist layer, so its actual realization is substantially considered as being impossible.
Meanwhile, there is recently proposed a method for applying laser interference lithography after forming a diffraction mask on the photoresist layer in order to further decrease the pitch when a fine pattern is formed using laser interference lithography. Generally, this method is classified into interfering 0th and −1st diffracted lights with each other or interfering ±1st diffracted lights with each other. However, the method using 0th and −1st diffracted lights allows formation of a fine pattern with a pitch identical to a period of the diffraction grating, and the method using ±1st diffracted lights allows formation of an interference pattern with a pitch that is ½ of a period of the diffraction grating.
Thus, as the period of the diffraction grating is smaller, a finer pattern may be formed. However, due to the physical limit, it is practically impossible to make a diffraction mask having a grating period that is ½ or less of the wavelength of the laser source.
In recent, immersion lithograph is more spotlighted in order to enhance resolution of laser interference lithography. The immersion lithography uses that a wavelength of electromagnetic wave is shortened in a medium with a high refractive index, and a prism is frequently used.
FIG. 3 is a schematic view for illustrating the conventional immersion interference lithography using a prism.
As shown in FIG. 3, in case a prism 4 is used, the prism 4 is added onto a photoresist layer 2 formed on an upper surface of a work substrate 1, and then the immersion interference lithography is conducted. The immersion interference lithography using the prism 4 forms an interference pattern using two incident lights A perpendicularly inputting to a speculum of the prism 4. In case there is no prism, the period of the interference pattern becomes λ/2 sin θ. However, if the prism 4 is used, assuming that the prism 4 has a refractive index of n, the inside wavelength becomes λ/n, so the pattern period is decreased to λ/2 n sin θ, which further enhances resolution of the fine pattern rather than an existing method. However, in case of the immersion interference lithography using the prism 4, an index matching liquid 3 should be used for matching of refractive indexes between the prism 4 and the photoresist layer 2. if the index matching liquid 3 is not used, an air gap may occur between the prism 4 and the photoresist layer 2, so stains may be created on the exposed pattern. In addition, since the intensity of light causing constructive interference is decreased due to the total reflection in the prism 4, the degree of definition of the fine pattern is deteriorated.
Also, the immersion interference lithography using a prism demands an additional device for immersion, so the equipment for alignment of optical systems are required and thus it is not suitable for mass production. In case a large prism is required, the laser lithography equipment should be also enlarged. Further, a laser source with high coherence should be used similarly to the existing laser interference lithography, which is also designated as an obstacle in mass production.